Complex impedance spectrometry is used in a variety of applications including transmission line and biomedical tissue analysis. Impedance spectrometry measurements are either made in the time domain or the frequency domain. In the time domain, impedance spectrometry is performed by exciting the sample or system to be analyzed with an electric current pulse and measuring the resulting deformed voltage pulse. The complex impedance spectrum can then be calculated by applying a Fast-Fourier Transform or other spectral analysis algorithms to the measured pulse time series. The basic method was described by P. J. Hyde in 1970, and further embodiments were described in U.S. Pat. No. 5,633,801 to Bottman and U.S. Pat. No. 5,454,377 to Dzwonczyket al. This method is limited at high frequencies, however, to the rise time and sampling frequency of the electronics used, and is disadvantageous in that the electronic circuit design becomes difficult for bandwidths larger than 1 MHz.
Impedance spectrometry in the frequency domain involves making measurements at different frequencies, either by sweeping the frequency (as in expensive impedance gain/phase analyzers), or by making discrete measurements at selected frequencies to estimate the impedance spectrum. To simplify the required instrumentation, several methods have been described. For example, one proposed system, as set forth in B. Rigaud et al., “Tissue Characterization by Impedance: A Multifrequency Approach,” Physiol. Meas. 15(2A), A13–A20 (1994), involves using parallel amplification and digital conversion circuits to allow for the simultaneous measurement of voltages generated by a variable frequency excitation current source in the sample to be analyzed and in a reference resistance. The captured signals are then processed using a personal computer. Impedance measurements by this system are accurate because the impedance is calculated relative to the reference signal. However, this system requires precise synchronization between the two A/D channels to facilitate accurate phase measurements, and the bandwidth of the system is limited by the sampling frequency of the A/D converters.
For higher bandwidth impedance spectrometry, another proposed system, as set forth in R. Pallas-Areny & J. G. Webster, “Bioelectric Impedance Measurements Using Synchronous Sampling,” IEEE Trans. on Biomed. Eng., vol. 40 no. 8, pp. 824–829 (August 1993), utilizes a synchronous demodulation and quadrature sampling method. In this method, the voltage from the excited sample is demodulated to a preset low frequency, independent of the excitation frequency, simplifying the requirements of the sampling and conversion circuit. To retain the phase information, the demodulation and the sampling must be synchronized. The sampling is triggered by a synchronization circuit such that the measurements are made in-phase (0°) and quadrature (90°). This system was further simplified in U.S. Pat. No. 5,807,272 to Kun et al., by using a synchronization circuit accurate at high frequencies and eliminating the need for demodulation. However, synchronization circuits can be expensive and/or difficult to implement, rendering systems that require them less than fully ideal.
Accordingly, it is a primary object of the present invention to provide a high-bandwidth system for accurately measuring the complex impedance spectrum of tissue or other objects, without the need for synchronization circuits or other expensive electronics.
Another primary object of the present invention is to provide a method for high-bandwidth complex impedance spectrometry that is easily and inexpensively implemented using commonly available integrated circuits and microprocessors/personal computers.
Another object of the present invention is to provide a system for measuring complex impedance spectra that simplifies the required hardware, and/or improves the performance of other previously reported methods.